Textures with alpha:

This is good for patches of texture. You need to worry about the edges of the field. If you have a plane partially covering the screen along the edge of the plane will look strange because it will be a hard edge. One way to get around this is use an image-map with alpha to create an irregular shape. You will also probably want to use multiple layers of this technique for images at different distances.

Basic Textures with alpha:

Create a template image with something(I used the gimp) template.jpg
Create a Plane in your scene and give it a simple cloud texture using alpha tex.jpg
You should have something like this textured plane
Add our template as a stencil to the texture stencil
Finally we have city with alpha

Things to try:

  • Use an empty to animate the textures, loc,size,rotation.
  • Use key-frames on the materials settings.
  • Duplicate your object and offset or scale it slightly larger then apply a mostly transparent material to it to get around the 8 texture limit.

More info can be found: